LTC1503-1.8/LTC1503-2
APPLICATIO S I FOR ATIO
S5
φ2
C1+
S1
φ1
VIN
VOUT
C1
(EXTERNAL)
C1–
S4
φ2
S7
S2
φ2
φ1
C2 +
C2
(EXTERNAL)
C2 –
S6
φ2
S3
φ1
GND
1503-1.8/2 F01b
Figure 1b. Step-Down Charge Pump in 3-to-2 Mode
In 1-to-1 mode, switch S1 and S2 are connected in series
between VIN and VOUT as needed to boost VOUT back into
regulation (see Figure 1c). The REG ENABLE signal from
the main comparator (COMP1) controls switches S1 and
S2 directly. Since all of the VOUT current is sourced from
VIN, the efficiency in 1-to-1 mode is approximately equal
to that of a linear regulator.
S2
C1+
S1
VIN
VOUT
C1
(EXTERNAL)
C1–
1503-1.8/2 F01c
Figure 1c. Step-Down Charge Pump in 1-to-1 Mode
Mode Selection and Mode Skipping
The optimal step-down conversion mode is chosen based
on VIN to VOUT differential voltage and output load condi-
tions. Two internal comparators are used to select the
default step-down mode based on the VIN and VOUT
voltage. A separate comparator (COMP2) is used to sense
a droop on VOUT due to a heavy output load and force the
charge pump to skip to a higher output current mode to
maintain regulation. This will only occur as VIN/VOUT nears
a 3-to-2 or 1-to-1 transition point. For example, under light
load conditions, the LTC1503-X can operate in 2-to-1
mode when VIN equals 4.1V with greater than 90% effi-
ciency. However, when the load is increased, the part can
no longer supply enough output current in 2-to-1 mode to
maintain regulation. This causes VOUT to droop below the
regulation point until COMP2 trips and forces the part to
skip from 2-to-1 mode to 3-to-2 mode. The COMP2
threshold is about 17mV (VOUT referred) below the main
comparator regulation point. Hysteresis in COMP2 will
force the part to transition in and out of mode skipping.
This will result in a slight VOUT decrease of approximately
20mV under mode skipping conditions.
Shutdown/Soft-Start Operation
The SHDN/SS pin is used to implement both low current
shutdown and soft-start. The soft-start feature limits
inrush currents when the regulator is initially powered up
or taken out of shutdown. Forcing a voltage lower than
0.35V (typ) will put the part into shutdown mode. Shut-
down mode disables all control circuitry and forces the
charge pump VOUT into a high impedance state. A 2µA pull-
up current on the SHDN/SS pin will force the part into
active mode if the pin is left floating or is driven with an
open-drain output that is in a high impedance state. If the
pin is not driven with an open-drain device, it must be
forced to a logic high voltage of 2.2V (min) to ensure
proper VOUT regulation. The SHDN/SS pin should not be
driven to a voltage higher than VIN.
To implement soft-start, the SHDN/SS pin must be driven
with an open-drain device and a capacitor must be
connected from the SHDN/SS pin to GND. Once the open-
drain device is turned off, a 2µA pull-up current will begin
charging the external SS capacitor and force the voltage
on the pin to ramp towards VIN. As soon as the SHDN
threshold is reached (0.35V typ), the internal reference
voltage which controls the VOUT regulation point will
follow the ramp voltage on the SHDN/SS pin (minus a
0.35V offset to account for the SHDN threshold) until the
reference reaches its final band gap voltage. This occurs
when the voltage on the SHDN/SS pin reaches
7